Ground fault circuit interrupters (GFCI) are widely used in residential circuits as protective devices to prevent potentially lethal electric shock to appliance users in the event of a ground fault. GFCI devices, in response to a differential in the current flowing in the line and neutral conductors of a load circuit indicative of a ground fault, energize a solenoid which then acts via a trip mechanism to open contacts and thus interrupt the circuit. Circuit interruption is achieved with requisite speed such that the flow of ground fault current through a person's body is halted before any injury is inflicted.
One configuration of a GFCI is that of a receptacle installed in a wall outlet box. As compared to a non-ground fault receptacle, a typical GFCI receptacle includes many additional components, such as pairs of fixed and movable contacts, a trip/reset mechanism, a solenoid, a differential current transformer, a ground neutral transformer, an electronic circuit board, internal wiring, etc. Since a GFCI receptacle must fit in a standard size outlet box, these components must be miniaturized and densely packaged to achieve a compact design conducive to facile installation even for the do-it-yourself homeowner.
One of the more spacious components in a GFCI receptacle is the solenoid which acts to defeat a latch in the trip/reset mechanism and allow the circuit interrupting contacts to spring open. Typically, the solenoid plunger is biased by a spring to a quiescent or return position in spaced relation to the trip latch. When a circuit interruption is called for, the solenoid coil is energized to magnetically drive the plunger to an extended position, in the process striking the latch to release the trip/reset mechanism and open the contacts. Since the magnetic force on the plunger must overcome the plunger return spring bias, the magnetic circuit of the solenoid must be fairly robust, thus adding size and cost.
The trip/reset mechanism is another component that make significant contributions to the size and cost of a GFCI receptacle. This mechanism must handle the trip and reset functions, and also must be designed to defeat any attempt to manually close or hold closed the contacts in presence of a ground fault, such as by continued depression of the mechanism reset button. To accommodate these various functions, the typical trip/reset mechanism design calls for a multiplicity of intricate parts representing significant manufacturing costs.